The present invention relates to the field of precision force measurements systems. In particular, a system for automatic calibration of high precision force balances is provided.
Force balances consisting of strain gauges are widely used to measure forces encountered on a object when it is placed within a test environment. These balances are commonly used to measure three components of aggregate force: axial force, side force, normal (vertical) force, as well as rolling, yawing, and pitching moments. The use of force balances is widespread and includes applications in component stress tests including aerodynamic wind tunnel testing of loads.
Force balances are usually calibrated manually using a complex system of free hanging precision weights. Although this method is extremely accurate, the process is very complex and labor intensive, requiring from two to three man-weeks to complete a single calibration. Additionally, the use of gravity based loading causes difficulties when applying loads in all three orthogonal axes simultaneously--often leading to the development of large lever, cable and pulley based systems or a requirement that the entire calibration system be rotated.
In 1962, the National Aeronautics and Space Administration acquired a semi-automatic method for calibrating force balances. Similar designs were also developed by Carl Schenck AG (1989) in Germany and Israel Aircraft Industries (1991) in Israel and have subsequently been made commercially available in the United States. Each of these systems reduced the time involved in calibrating a balance to a single work shift (approximately eight man-hours); however, the new designs still had significant disadvantages.
First, each of the designs is quite large, complex and expensive. These calibration systems are not portable and must be installed at a fixed location.
Additionally, the system accuracy involved in these systems is difficult to determine. Since system accuracy is based on the accuracy of very high precision load cells, any complex load path from the cells and the force generators to the balance being calibrated makes alignment, deflection, and friction (or the changes in these properties) extremely critical factors in overall system performance. All of the automatic systems in the prior art can only infer system accuracy of between 0.1% and 0.2% by comparing their calibrations of test balances with calibrations performed using traditional gravity loads.